This is a tutorial on how to setup calling to/from MS LCS 2005 using SER and Asterisk. In this case LCS is acting as the IM/Presence/VoIP server, SER converts protocols TCP2UDP and Asterisk is a SIP to ISDN gateway. This setup allows for outbound calls from Microsoft Office Communicator – OC to a normal telephone and inbound calls from a normal telephone to Microsoft Office Communicator - OC. This also allows you to access services provided by Asterisk such as Meetme Conferencing. You can also use Windows Messenger 5.1 instead of OC client.
You can find the tutorial here.